The use of retention systems is well known in papermaking processes. They have the function of improving the retention (that is to say the quantity of filler in the paper) and the drainage (that is to say, the dewatering rate) during the manufacture of the sheet.
Patent EP 1 328 161 describes a system for improving retention and drainage during the manufacture of paper or board using three retention aids. The first is a cationic flocculant having an intrinsic viscosity IV above 4 dl/g, the second is a siliceous material and the third a water-soluble anionic polymer having an IV of 4 dl/g or more.
All the retention and drainage systems known in the prior art are characterized by the fact that as the main retention aid, they use water-soluble polymers having high molecular weight, above 1 million g/mol, generally above 3 million, called flocculants. They are generally cationic and, owing to their high molecular weight, have the property of occurring in the form of an emulsion (reverse), microemulsion, powder or dispersion.
The Hofmann degradation reaction on a base (co)polymer is a known reaction for converting an amide to a primary amine having one less carbon atom.
Hofmann degradation products are well known for their use as dry strength agents. In practice, the molecular weight of the degradation product is generally less than 1 million g/mol, hence much lower than the molecular weight of the cationic polymers used as drainage and retention aids (above 2 million g/mol). When used as strength agents in papermaking processes, they are combined with low molecular weight anionic resins.
Such a system is, for example, the one described in document WO2006/075115 from the Applicant. This relates to a cationic polymer obtained by Hofmann degradation reaction, produced in a concentration above 3.5% combined with an anionic resin of which the highest viscosity is 9000 cps (15% solution), which corresponds to a maximum IV of about 2.0 dl/g. A similar system is also described in document WO2008/107620, also from the Applicant, which is distinguished from the former in that the base copolymer on which the degradation is carried out is branched, and in that the degradation is carried out in the presence of calcium hypochlorite. In this document, the maximum viscosity described of the anionic resin is 2500 cps, which corresponds to a maximum IV of 1.6 dl/g. Application WO2009/013423, also from the Applicant, is distinguished from the preceding documents in that the polymer obtained by the Hofmann degradation reaction is branched after the said reaction. As previously, the IV of the anionic resin used does not exceed 1.6 dl/g.
It is essential in the invention to clearly distinguish the retention and drainage properties on the one hand, and the dry strength properties on the other hand.
Retention properties mean the ability to retain the suspended matter in the paper pulp (fibres, fines, fillers (calcium carbonate, titanium oxide), etc.) on the preparation web, hence in the fibrous mat which constitutes the final sheet. The action mechanism of the retention aids is based on the flocculation of this suspended matter in the water. This ensures that the flocs formed are more easily retained on the preparation web.
As to the drainage properties, they represent the ability of the fibrous mat to remove or drain the maximum of water so that the sheet dries as rapidly as possible.
Since these two properties (retention and drainage) are intimately linked, as one depends on the other, the aim is to find the best compromise between retention and drainage. In general, the person skilled in the art refers to a retention and drainage aid, because the same types of product serve to improve these two properties.
They are generally slightly cationic high molecular weight polymers (at least 1 million g/mol). These polymers are generally introduced in a proportion of 50 to 800 g/t of dry polymer with respect to the dry paper.
The points of introduction of these aids in the papermaking process are generally located in the short circuit, that is to say, after the fan pump, and hence in thin stock, of which the concentration is generally lower than 1% by weight of dry matter, usually between 0.1 and 0.7%.
Contrary to the retention and drainage properties, the dry strength represents the ability of the sheet to withstand the mechanical stresses and damage such as perforation, tearing, tension, delamination and various forms of compression. These relate to the final properties of the sheet.
Dry strength resins are generally medium molecular weight polymers (10,000 to 1,000,000 g/mol), and the usual dosages applied are from 1.5 to 2 kg/t (dry polymer with respect to dry paper), that is to say, 5 to 10 times higher than the dosages applied to retention and drainage, even though a wide range between 100 and 20,000 g/t is disclosed in application WO2009/013423.
Furthermore, the points of introduction of these dry strength resins, in particular for the cationic polymer, are generally located in thick stock, of which the dry matter concentration is generally above 1% and usually above 2%, hence before the fan pump, and therefore the dilution with the white water.
The Applicant further indicates that the examples in application WO2009/13423 mention pulp concentrations of about 0.3 to 0.5%, which correspond to the values required to perform standard laboratory tests, but which do not correspond to the pulp concentrations in industrial processes in which dry strength agents are used, and which are generally above 2% of dry matter.
The polymers providing dry strength are joined to the fibres by a hydrogen and/or ionic bond so that, once the sheet is dried, the mechanical strength of the paper is improved.
It therefore goes without saying that, on the one hand, good retention and drainage properties are recommended to optimise the manufacture of the paper and hence the productivity of the paper machine, and on the other hand, in a totally different manner, good dry strength properties will have the effect of improving the mechanical properties (and hence the quality) of the sheet.
In the rest of the description and in the claims, all the polymer dosages expressed in g/t are given as weight of active polymer per tonne of dry pulp.
The dry strength of the paper is, by definition, the strength of the normally dried sheet. The values of the burst and tensile strength conventionally provide a measure of the dry strength of the paper.
A side effect of the application of these dry strength systems in high dosages, is accompanied, subsidiarily, by an improvement in retention, but at prohibitive costs, which cannot possibly justify their use for this purpose alone.
It therefore appears from the above discussion that it was known on the filing date of the present application how to combine, in order to improve the dry strength in the process for manufacturing paper or board, a low molecular weight cationic Hofmann degradation product with an anionic resin also having a low molecular weight, the two agents being introduced during the process in doses of about 1.5 to 2 kg/t.
Despite the progress achieved in recent years, the paper industry still faces the following problems in retention and drainage systems:                difficulty and cost of applying cationic flocculants as main retention aid. Their high molecular weight entails their use in forms demanding preparation units (emulsion reversal, powder dissolution), costly in manpower, equipment and maintenance. The necessary filtration steps are also the cause of many line shutdowns and added costs;        a problem of filtration of insoluble particles, and even clogging of the filters, can cause major defects in the paper machine: breakage, defects in the paper such as patches, holes, etc.;        the negative impact on the formation of the sheet, during the use of excessively high molecular weight polymers or high molecular weight polymers in high dosages;        the use of high molecular weight flocculant necessitated by increasingly high machine speeds hence increasingly higher sheet shear and filler content.        